This invention relates to an RF probe of a magnetic resonance imaging (MRI) apparatus.
A cross system RF probe using separate transmission and reception coils has ordinarily been used as an RF transmission/reception system for MRI. This is the system which uses exclusive RF coils for RF magnetic field transmission and MR signal reception, respectively. A saddle coil, a slotted tube resonator (hereinafter abbreviated to "STR"), a multiple element resonator (MER; sometimes referred to also as "birdcage resonator"), etc., are used for the transmission coil. A surface coil is used for ultra-high speed imaging of the heart. The STR and the MER are used as the reception coil for imaging the head, and a round coil is used for imaging the spine and a local part. In order to allow the reception coil to stably operate, an inductive coupling system using a pickup coil (PUC) for RF reception is used in a high magnetic field MRI apparatus of 1.5 T, for example. A QD system is used to improve a reception sensitivity, as described, for example, in JP-A-4-17837. The birdcage resonator is described, for example, in JP-A-61-95234 and JP-A-60-132547.
FIG. 1A of the accompanying drawings illustrates the structure of a cross system RF probe according to the prior art. Reference numerals 101 denotes a transmission coil and reference numeral 100 does a reception coil. The reception coil comprises a resonator 6 and a pickup coil (PUC) 103.
When the transmission coil 101 generates a strong RF magnetic field 102 as shown in FIG. 1A, an induction current 105 flows through the reception coil 100. In the RF probe according to the prior art, the RF magnetic field is offset by this induction current 105 and heterogeneity occurs in the RF magnetic field, so that image quality of the MRI image deteriorates. To prevent this problem, a decoupling circuit using a diode is employed for the resonator 6 and the PUC 103. The decoupling circuit 104 shown in FIG. 1A is such an example. More definitely, the decoupling circuit comprises a cross diode pair 106 or a trap circuit including diodes shown in FIG. 1B.
Recently, however, a sequence for obtaining a high quality image at a high speed by applying a strong RF magnetic field of about 15 KW, for example, has gained a wide application, and an instantaneous current value flowing through the decoupling circuit 104 becomes great. Accordingly, a selection standard for the diodes 106 used for the decoupling circuit 104 has become more severe. Under such a circumstance, the problem has often arisen in that diode characteristics deteriorate with the passage of time and the MRI image deteriorates as well. In the inductive coupling system, in particular, elimination of the induction current 105 of the PUC 103 is important. However, if a decoupling circuit 104 having a large power capacity is connected to eliminate the induction current, a signal-to-noise ratio (S/N) drops.